To elucidate the mechanism of intracellular protein turnover, we have been attempting to isolate from bacteria a system that will degrade normal cell proteins. Glutamine synthetase from E. coli is proteolyzed by bacterial cell extracts once it has been oxidatively modified by a model inactivating system consisting of ascorbate, iron and oxygen. The purpose of this project is to purify a proteolytic activity, presumably a protease, that will preferentially degrade the inactive versus the native glutamine synthetase. Starting from E. coli whole cell extracts, a proteolytic activity has been partially purified which degrades the inactive glutamine synthetase 10 times more rapidly than the native. The proteolytic activity is soluble rather than membrane associated, precipitates in a 60-70% ammonium sulfate cut, does not bind DEAE ion exchanger at pH 7.5, is retained by an ultrafiltration membrane of 50,000 molecular weight pore size, and elutes ahead of the protein peak on gel filtration columns.